This invention relates to a method for joining active metals, alloys of active metals, composites formed from active metals or alloys of active metals, or combinations thereof.
As used herein the term "active metals" means the metals niobium, tantalum, zirconium, vanadium, titanium, molybdenum, tungsten, and hafnium. The active metals possess a number of common physical, chemical, and engineering properties; the principal ones being high melting point, chemical activity in the liquid and solid states at elevated temperatures with atmospheric gases, susceptibility to embrittlement, and a high-rate of grain growth when heated above the recrystalization temperature.
As used herein, the term "active metal substrates" means the articles and members formed from the active metals, alloys of the active metals, or composites formed from the active metals or alloys of active metals. For example, active metal substrates are shown in U.S. Pat. No. 4,775,547 where titanium alloy composites are reinforced with silicon carbide filaments. It is disclosed in the U.S. Pat. No. 4,775,547 that the silicon carbide reinforced titanium alloy composites have a high strength-to-weight ratio, making them attractive for use in aircraft engines having high thrust-to-weight ratios, aircraft structures, as well as in wound rotors, casings, and other intermediate temperature high stress applications.
Many applications for the use of such active metal substrates will require joining of the active metal substrates. For instance, titanium alloys will be joined to other titanium alloys, or titanium alloys will be joined to silicon carbide reinforced titanium alloy composites. The ductility and toughness of the joints and the joined active metal substrates depends to a marked extent on the content of interstitial impurity elements introduced into the substrates during the joining process. At elevated temperatures the active metals readily form oxides, hydrides, and nitrides which embrittle the metal. Therefore, in joining active metal substrates it is important to protect the substrates from exposure to interstitial elements such as oxygen, nitrogen, and hydrogen, and to keep the content of interstitial impurity elements in the joint and substrates to a minimum.
One successful method known for joining active metal substrates is vacuum diffusion bonding. Vacuum diffusion bonding is a pressure welding process where pieces to be joined are placed in a vacuum, heated, and pressed to cause the edges of the pieces to move within the range of atomic forces. The interdiffusion of atoms across the interface between the two substrates causes the bonding. Strong and ductile joints can be formed because the metals are protected from exposure to interstitial elements, and the temperature and degree of substrate deformation during the bonding cycle can be controlled to provide desired microstructures in the substrates and joints. However, the surfaces of the substrates, and the roughness of the surfaces must be carefully prepared to provide intimate contact and mating of the surfaces to be joined during the bonding.
It is an object of this invention to provide a method for joining active metal substrates by radio frequency low-pressure plasma-spraying.
Another object of this invention is to provide a method for joining active metal substrates while minimizing the introduction of interstitial impurity elements in the joint.
Another object of this invention is to provide a method for joining active metal substrates without degrading the strength and toughness properties of the substrate.
Another object of this invention is to provide a method for joining active metal substrates by a strong diffusion bond, without intimate contact and mating of the substrate surfaces to be joined.